In a laser gyroscope, or laser angular rate sensor (LARS), a pair of laser light beams (monochromatic radiation produced or generated by gas discharge) travel about a closed loop path. The two beams, traveling in opposite directions around the loop, are compared in frequency. When the system of which the LARS is a component is at rest in inertial space, then the two beams oscillate at the same or substantially the same frequency since they see equal path lengths. If the system rotates in inertial space, the two oppositely traveling beams see unequal paths, oscillate at different frequencies, and this frequency difference may be measured as an indication of the direction and amount of rotation. A typical means for measuring the frequency difference is the optical interferometer fringe pattern measuring technique well known to those skilled in the art. It is also well known that for small rotation rates of the entire system, the two beams are inclined to resonate together and "lock-in". In the prior art several arrangements have been proposed to reduce this lock-in phenomenon. An example is U.S. Pat. No. 3,373,650 granted Mar. 19, 1968 in the name of J. E. Killpatrick and assigned to the same assignee as the present intention. This patent teaches electrical and/or mechanical oscillation or biasing or "dithering" of the LARS with respect to inertial space so that the LARS seem to be rotating at a rate higher than the lock-in rate . . . at least for a majority of the time. This invention was further enhanced by the invention disclosed in U.S. Pat. No. 3,467,472 also granted in the name of J. E. Killpatrick and assigned to the same assignee as the present invention; the teaching of this invention further reducing the lock-in error by randomizing the oscillation or biasing or dithering of the beams so that the small errors in the extremities of the oscillation are no longer cumulative.
It has been recognized, however, that notwithstanding the significant benefits derived from utilizing the teachings of the two mentioned prior art U.S. Pat. Nos. 3,373,650 and 3,467,472, there nevertheless still remain certain errors known by those skilled in the art as a random drift rate or random walk type error. The present invention is a significant improvement over prior art techniques and specifically greatly reduces the random walk type error in a LARS.
Scattering of light from one of the beams of the laser angular rate sensor into the other (oppositely directed) beam is considered to be the source of lock-in. Defects in the mirrors are considered to be the primary sources of such scattered light. Present LARS have a wide variation in their lock-in values . . . a 10:1 variation in the lock-in rate for one model of a LARS is not atypical. In order to achieve consistent and minimal lock-in values, it is presently necessary to use very costly manufacturing techniques which take the form of attempting to obtain nearly perfect ultralow scatter mirrors and a certain amount of trial and error in matching mirrors and laser cavities. These expensive manufacturing techniques are most undesirable and have been an obstacle in the path of the utilization of LARS in various systems applications.